Process for improving the emission of electron field emitters

ABSTRACT

This invention provides a process for improving the field emission of an electron field emitter comprised of an acicular emitting substance such as acicular carbon, an acicular semiconductor, an acicular metal or a mixture thereof, comprising applying a force to the surface of the electron field emitter wherein the force results in the removal of a portion of the electron field emitter thereby forming a new surface of the electron field emitter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Appl. No.60/213,002, filed on Jun. 21, 2000; U.S. Provisional Appl. No.60/213,159, filed on Jun. 22, 2000; and U.S. Provisional Appl. No.60/287,930, filed on May 1, 2001.

FIELD OF THE INVENTION

This invention relates to a process for improving the emission ofelectron field emitters, particularly electron field emitters comprisedof acicular carbon.

BACKGROUND OF THE INVENTION

Field emission electron sources, often referred to as field emissionmaterials or field emitters, can be used in a variety of electronicapplications, e.g., vacuum electronic devices, flat panel computer andtelevision displays, emission gate amplifiers, and klystrons and inlighting.

Display screens are used in a wide variety of applications such as homeand commercial televisions, laptop and desktop computers and indoor andoutdoor advertising and information presentations. Flat panel displayscan be an inch or less in thickness in contrast to the deep cathode raytube monitors found on most televisions and desktop computers. Flatpanel displays are a necessity for laptop computers, but also provideadvantages in weight and size for many of the other applications.Currently laptop computer flat panel displays use liquid crystals, whichcan be switched from a transparent state to an opaque one by theapplication of small electrical signals. It is difficult to reliablyproduce these displays in sizes larger than that suitable for laptopcomputers.

Plasma displays have been proposed as an alternative to liquid crystaldisplays. A plasma display uses tiny pixel cells of electrically chargedgases to produce an image and requires relatively large electrical powerto operate.

Flat panel displays having a cathode using a field emission electronsource, i.e., a field emission material or field emitter, and a phosphorcapable of emitting light upon bombardment by electrons emitted by thefield emitter have been proposed. Such displays have the potential forproviding the visual display advantages of the conventional cathode raytube and the depth, weight and power consumption advantages of the otherflat panel displays. U.S. Pat. Nos. 4,857,799 and 5,015,912 disclosematrix-addressed flat panel displays using micro-tip cathodesconstructed of tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350and WO 94-28571 disclose flat panel displays wherein the cathodes haverelatively flat emission surfaces.

Field emission has been observed in two kinds of nanotube carbonstructures. L. A. Chernozatonskii et al., Chem. Phys. Letters 233, 63(1995) and Mat. Res. Soc. Symp. Proc. Vol. 359, 99 (1995) have producedfilms of nanotube carbon structures on various substrates by theelectron evaporation of graphite in 10⁻⁵-10⁻⁶ torr(1.3×10⁻³-1.3×10⁻⁴Pa). These films consist of aligned tube-like carbonmolecules standing next to one another. Two types of tube-like moleculesare formed; the A-tubelites whose structure includes single-layergraphite-like tubules forming filaments-bundles 10-30 nm in diameter andthe B-tubelites, including mostly multilayer graphite-like tubes 10-30nm in diameter with conoid or dome-like caps. They report considerablefield electron emission from the surface of these structures andattribute it to the high concentration of the field at thenanodimensional tips. B. H. Fishbine et al., Mat. Res. Soc. Symp. Proc.Vol. 359, 93 (1995) discuss experiments and theory directed towards thedevelopment of a buckytube (i.e., a carbon nanotube) cold field emitterarray cathode. A. G. Rinzler et al., Science 269, 1550 (1995) report thefield emission from carbon nanotubes is enhanced when the nanotubes tipsare opened by laser evaporation or oxidative etching. W. B. Choi et al.,Appl. Phys. Lett. 75, 3129 (1999) and D. S. Chung et al., J Vac. Sci.Technol. B 18(2) report the fabrication of a 4.5 inch flat panel fielddisplay using single-wall carbon nanotubes-organic binders. Thesingle-wall carbon nanotubes were vertically aligned by paste squeezingthrough a metal mesh, by surface rubbing and/or by conditioning byelectric field. They also prepared multi-wall carbon nanotube displays.They note that carbon nanotube field emitters having good uniformitywere developed using a slurry squeezing and surface rubbing technique.They found that removing metal powder from the uppermost surface of theemitter and aligning the carbon nanotubes by surface treatment enhancedthe emission. Single-wall carbon nanotubes were found to have betteremission properties than multi-wall carbon nanotubes but single-wallcarbon nanotube films showed less emission stability than multi-wallcarbon nanotube films. Zettl et al., U.S. Pat. No. 6,057,637 Claim afield emitter material comprising a volume of binder and a volume ofB_(x)C_(y)N_(z) nanotubes suspended in the binder, where x, y and zindicate the relative ratios of boron, carbon and nitrogen.

N. M. Rodriguez et al., J. Catal. 144, 93 (1993) and N. M. Rodriguez, J.Mater. Res. 8, 3233 (1993) discuss the growth and properties of carbonfibers produced by the catalytic decomposition of certain hydrocarbonson small metal particles. The patents U.S. Pat. Nos.5,149,584,5,413,866, 5,458,784, 5,618,875 and 5,653,951 disclose uses for suchfibers.

There is a continuing need for technology enabling the commercial use ofacicular carbon in electron field emitters.

SUMMARY OF THE INVENTION

This invention provides a process for improving the field emission of anelectron field emitter comprised of an acicular emitting substance suchas acicular carbon, an acicular semiconductor, an acicular metal or amixture thereof, comprising applying a force to the surface of theelectron field emitter such that a portion of the electron field emitteris removed or rearranged thereby forming a new surface of the electronfield emitter.

In a preferred embodiment, this invention provides a process forimproving the field emission of an electron field emitter comprised ofacicular carbon, comprising:

-   -   (a) contacting a material with the electron field emitter,        wherein the material forms an adhesive contact with the electron        field emitter and the adhesive contact provides sufficient        adhesive force when the material is separated from the electron        field emitter so that a portion of the electron field emitter        adheres to the material thereby forming a new surface of said        electron emitter; and    -   (b) separating the material from the electron field emitter.

Carbon nanotubes are the preferred acicular carbon. Single wall carbonnanotubes are more preferred and laser ablation grown single wall carbonnanotubes are especially preferred. Preferred for use in this processare electron field emitters in which the carbon nanotubes are less thanabout 9 wt % of the total weight of the electron field emitter. Morepreferred are electron field emitters in which the carbon nanotubes areless than about 5 wt % of the total weight of the electron fieldemitter. Still more preferred are electron field emitters in which thecarbon nanotubes are less than about 1 wt % of the total weight of theelectron field emitter. Most preferred are electron field emitters inwhich the carbon nanotubes are about 0.01 wt % to about 2 wt % of thetotal weight of the electron field emitter.

There is also provided a composition for use as a screen printable pastecontaining solids comprising carbon nanotubes, wherein the carbonnanotubes are less than 9 wt % of the total weight of solids in thepaste. More preferred is the composition wherein the carbon nanotubesare less than 5 wt % of the total weight of solids in the paste. Stillmore preferred is the composition wherein the carbon nanotubes are lessthan 1 wt % of the total weight of solids in the paste. Most preferredis the composition wherein the carbon nanotubes are about 0.01 wt % toabout 2 wt % of the total weight of solids in the paste. This paste isespecially useful in fabricating an electron field emitter which thenundergoes the improvement process of the invention. Such an emitter hasexcellent emission properties, good adhesion to the substrate along withthe advantages of ease of preparing and comparatively low cost ofmaterials and processing.

The improved electron field emitters are useful in flat panel computer,television and other types of displays, vacuum electronic devices,emission gate amplifiers, klystrons and in lighting devices. The processis especially advantageous for producing large area electron fieldemitters for flat panel displays, i.e., for displays greater than 30inches (76 cm) in size. The flat panel displays can be planar or curved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the emission results for the electron field emitter ofExample 1 with emission current density plotted as a function of appliedelectric field for the electron field emitter before and afterundergoing the process of the invention for improving emission.

FIG. 2 shows the emission results for the electron field emitter ofExample 2 with emission current density plotted as a function of appliedelectric field for the electron field emitter before and afterundergoing the process of the invention for improving emission.

FIG. 3 shows the emission results for the electron field emitter ofExample 3 with emission current plotted as a function of applied voltagefor the electron field emitter before and after undergoing the processof the invention for improving emission.

FIG. 4 shows the emission results for the electron field emitter ofExample 4 with emission current density as a function of appliedelectric field for the electron field emitter before and afterundergoing the process of the invention for improving emission.

FIG. 5 shows the emission results for the electron field emitter ofExamples 9-11 with emission current density plotted as a function ofapplied electric field for the electron field emitter before and afterundergoing the process of the invention for improving emission.

FIG. 6 has photographs of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitters ofExamples 9-11 before and after undergoing the process of the inventionfor improving emission.

FIG. 7 shows the emission results for the two electron field emitterscreated in Example 7 with emission current density plotted as a functionof applied electric field for both electron field emitters.

FIG. 8 shows the use of a thermally softened polymer in the process ofthe invention.

FIG. 9 has photographs of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitters ofExamples 13 and 14 after undergoing the process of the invention forimproving emission.

FIG. 10 is a photograph of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitter of Example15 after undergoing the process of the invention for improving emission.

FIG. 11 shows layers forming the fully screen-printed field emissivetriode of Example 16 that underwent the process of the invention forimproving emission.

FIG. 12 has photographs of the light emitted from a phosphor layerimpinged by electron emission from the screen-printed triode array ofExample 16 for both the diode and triode mode.

FIGS. 13A and 13B show graphene platelets and the fiber axis thereof.

FIG. 14 shows an embodiment of a process for improving an electron fieldemitter.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a process for improving the field emission of anelectron field emitter that is especially effective when the electronfield emitter is comprised of acicular emitting substances, e.g.,carbon, semiconductor, metal or mixtures thereof. As used herein,“acicular” means particles with aspect ratios of 10 or more. As usedherein, “electron field emitter” means the acicular emitting substanceand the glass frit, metallic powder or metallic paint or a mixturethereof used to attach the acicular emitting substance to a substrate.Therefore, as used herein, the “total weight of the electron fieldemitter” means the total weight of the acicular emitting substance andthe glass frit, metallic powder or metallic paint or a mixture thereofused to attach the acicular emitting substance to a substrate. The totalweight does not include the weight of the substrate which supports theelectron field emitter.

The acicular carbon can be of various types. Carbon nanotubes are thepreferred acicular carbon and single wall carbon nanotubes areespecially preferred. The individual single wall carbon nanotubes areextremely small, typically about 1.5 nm in diameter. The carbonnanotubes are sometimes described as graphite-like, presumably becauseof the sp² hybridized carbon. The wall of a carbon nanotube can beenvisioned as a cylinder formed by rolling up a graphene sheet.

Carbon fibers grown from the catalytic decomposition ofcarbon-containing gases over small metal particles are also useful asthe acicular carbon. As used herein, “catalytically grown carbon fibers”means carbon fibers grown from the catalytic decomposition ofcarbon-containing gases over small metal particles, each of which carbonfibers has graphene platelets arranged at an angle with respect to thefiber axis so that the periphery of the carbon fiber consistsessentially of the edges of the graphene platelets. The angle may be anacute angle or 90°.

Other examples of acicular carbon are polyacrylonitrile-based(PAN-based) carbon fibers and pitch-based carbon fibers.

Various processes can be used to attach acicular carbon to a substrate.The means of attachment must withstand and maintain its integrity underthe conditions of manufacturing the apparatus into which the fieldemitter cathode is placed and under the conditions surrounding its use,e.g. typically vacuum conditions and temperatures up to about 450° C. Asa result, organic materials are not generally applicable for attachingthe particles to the substrate and the poor adhesion of many inorganicmaterials to carbon further limits the choice of materials that can beused. A preferred method is to screen print a paste comprised ofacicular carbon and glass frit, metallic powder or metallic paint or amixture thereof onto a substrate in the desired pattern and to then firethe dried patterned paste. For a wider variety of applications, e.g.,those requiring finer resolution, the preferred process comprises screenprinting a paste which further comprises a photoinitiator and aphotohardenable monomer, photopatterning the dried paste and firing thepatterned paste.

The substrate can be any material to which the paste composition willadhere. If the paste is non-conducting and a non-conducting substrate isused, a film of an electrical conductor to serve as the cathodeelectrode and provide means to apply a voltage to and supply electronsto the acicular carbon will be needed. Silicon, a glass, a metal or arefractory material such as alumina can serve as the substrate. Fordisplay applications, the preferable substrate is glass and soda limeglass is especially preferred. For optimum conductivity on glass, silverpaste can be pre-fired onto the glass at 500-550° C. in air or nitrogen,but preferably in air. The conducting layer so-formed can then beover-printed with the emitter paste.

The emitter paste used for screen printing typically contains acicularcarbon, an organic medium, solvent, surfactant and either low softeningpoint glass frit, metallic powder or metallic paint or a mixturethereof. The role of the medium and solvent is to suspend and dispersethe particulate constituents, i.e., the solids, in the paste with aproper rheology for typical patterning processes such as screenprinting. There are a large number of such mediums known in the art.Examples of resins that can be used are cellulosic resins such as ethylcellulose and alkyd resins of various molecular weights. Butyl carbitol,butyl carbitol acetate, dibutyl carbitol, dibutyl phthalate andterpineol are examples of useful solvents. These and other solvents areformulated to obtain the desired viscosity and volatility requirements.A surfactant can be used to improve the dispersion of the particles.Organic acids such oleic and stearic acids and organic phosphates suchas lecithin or Gafac® phosphates are typical surfactants.

A glass frit that softens sufficiently at the firing temperature toadhere to the substrate and to the acicular carbon is required. A leador bismuth glass frit can be used as well as other glasses with lowsoftening points such as calcium or zinc borosilicates. Within thisclass of glasses, the specific composition is generally not critical Ifa screen printable composition with higher electrical conductivity isdesired, the paste also contains a metal, for example, silver or gold.The paste typically contains about 40 wt % to about 80 wt % solids basedon the total weight of the paste. These solids comprise acicular carbonand glass frit and/or metallic components. Variations in the compositioncan be used to adjust the viscosity and the final thickness of theprinted material.

The emitter paste is typically prepared by three-roll milling a mixtureof acicular carbon, organic medium, surfactant, a solvent and either lowsoftening point glass frit, metallic powder or metallic paint or amixture thereof. The paste mixture can be screen printed usingwell-known screen printing techniques, e.g. by using a 165-400-meshstainless steel screen. The paste can be deposited as a continuous filmor in the form of a desired pattern. When the substrate is glass, thepaste is then fired at a temperature of about 350° C. to about 550° C.,preferably at about 450° C. to about 525° C., for about 10 minutes innitrogen. Higher firing temperatures can be used with substrates whichcan endure them provided the atmosphere is free of oxygen. However, theorganic constituents in the paste are effectively volatilized at350-450° C., leaving the layer of composite comprised of acicular carbonand glass and/or metallic conductor. The acicular carbon appears toundergo no appreciable oxidation or other chemical or physical changeduring the firing in nitrogen.

If the screen-printed paste is to be photopatterned, the paste containsa photoinitiator, a developable binder and a photohardenable monomercomprised, for example, of at least one addition polymerizableethylenically unsaturated compound having at least one polymerizableethylenic group.

A preferred composition for use as a screen printable paste is onecontaining solids comprising carbon nanotubes, wherein the carbonnanotubes are less than 9 wt % of the total weight of solids in thepaste. More preferred is the composition wherein the carbon nanotubesare less than 5 wt % of the total weight of solids in the paste. Stillmore preferred is the composition wherein the carbon nanotubes are lessthan 1 wt % of the total weight of solids in the paste. Most preferredis the composition wherein the carbon nanotubes are about 0.01 wt % toabout 2 wt % of the total weight of solids in the paste. This paste isespecially useful in fabricating an electron field emitter which is apreferred embodiment of an electron field emitter to undergo theimprovement process of the invention. These compositions with a lowconcentration of carbon nanotubes provide an excellent electron fieldemitter after the electron field emitter undergoes the improvementprocess. Typically, a paste comprising carbon nanotubes, silver andglass frit will contain about 0.01-6.0 wt % nanotubes, about 40-75 wt %silver in the form of fine silver particles and about 3-15 wt % glassfrit based on the total weight of the paste.

Electron field emitters can undergo the improvement process before orafter the firing step discussed previously; however, it is preferred tohave them fired before undergoing the process.

The process for improving the field emission of an electron fieldemitter comprised of an acicular emitting substance such as acicularcarbon, an acicular semiconductor, an acicular metal or mixtures thereofcomprises applying a force to the surface of the electron field emitterin a direction essentially normal to the plane of the electron fieldemitter such that a portion of the electron field emitter is removed orrearranged thereby forming a new surface of the electron field emitter.It is believed that the newly formed surface of the electron fieldemitter has acicular particles protruding from it.

One embodiment of the process can be envisioned as a fracturing of theelectron field emitter to provide a new emitting surface. This aspect ofthe process has been demonstrated by screen printing two electron fieldemitters comprised of acicular carbon onto two separate substrates. Asandwich structure was then formed by contacting the two screen-printedelectron field emitters so that the substrates formed the two outerlayers. The structure was then fired as described previously so that thetwo screen-printed electron field emitters formed a single fired emittersandwiched between the two substrates. The substrates were then pulledapart, fracturing the electron field emitter material. The two electronfield emitters showed improved emission properties over electron fieldemitters prepared by simply screen printing and firing.

A preferred embodiment of the process of this invention has beendemonstrated with electron field emitters comprised of acicular carbon.A material is contacted with the electron field emitter. The materialforms an adhesive contact with the electron field emitter and theadhesive contact provides sufficient adhesive force when the material isseparated from the electron field emitter so that a portion of theelectron field emitter is removed thereby forming a new surface of theelectron field emitter. The material is then separated from the electronfield emitter. Under certain conditions, when the material which formsthe adhesive contact is separated from the electron field emitter,little or none of the electron field emitter is removed but the surfaceof the electron field emitter is rearranged to form a new surface andthe newly formed surface of the electron field emitter has acicularparticles protruding from it. While the material which forms theadhesive contact with the electron field emitter is in contact with theelectron field emitter there is no translational motion by the materialwith respect to the electron field emitter. The only motion of thematerial occurs during the contacting and separating steps and thismotion is in direction essentially perpendicular to the plane of theelectron field emitter.

Any material that provides sufficient adhesive force can be used. Suchmaterial can be applied in solid or liquid form as a film or a coating.Such an adhesive force can be chemical, dispersive, electrostatic,magnetic, viscoelastic or mechanical in nature. The adhesive force maybe imparted by a separate step such as heating, light illumination, orlamination with or without applied pressure. Commercial adhesive tape isa readily available and convenient material especially for smallelectron field emitter surfaces. Any of the commercial transparent orinvisible tapes, masking tapes, duct tapes, sealing tapes, etc. can beused as the material to provide the adhesive force.

Pieces of tape can be contacted with and removed from the same electronfield emitter more then one time and each time the electron fieldemitter shows improved emission over the results obtained with electronfield emitters that had not undergone the process of the invention.

A thermally softened polymer film can also be used as the material thatprovides the adhesive contact with the electron field emitter. Such afilm is especially useful with large electron field emitter surfaces. Awide variety of polymers such as acrylics (e.g. Carboset® XPD-2264available from B. F. Goodrich Company, Charlotte, N.C.),ethylene/acrylic elastomers (e.g. Vamac®, available from E. I. du Pontde Nemours and Company, Wilmington, Del.), polyamides (e.g. the nylonmultipolymer resin Elvamide®, available from E. I. du Pont de Nemoursand Company, Wilmington, Del.), block co-polymer of styrene, butadiene,and isoprene (e.g. Kraton® available from Shell Chemical Company, ADivision of Sheel Oil Company, Houston, Tex.) and co-polymers ofethylene and vinyl acetate (e.g. Elvax® available from E. I. du Pont deNemours and Company, Wilmington, Del.), thermoplastic ethylenemethacrylic acid copolymers (e.g. Nucrel® available from E. I. du Pontde Nemours and Company, Wilmington, Del.), ionomers (e.g. Surlyn®available from E. I. du Pont de Nemours and Company, Wilmington, Del.),Bynel® CXA coextrudable adhesive resins available from E. I. du Pont deNemours and Company, Wilmington, Del. and mixtures thereof can be usedfor this purpose. The thermal and adhesive properties of the softpolymer can be further customized by blending with monomer, tackifiers,and plasticizers.

A metallic piece of adhesive tape was contacted with and removed from anelectron field emitter. A portion of the electron field emitter adheredto the metallic tape and when tested for emission showed emissionproperties improved over that of the electron emitter before undergoingthe process of the invention for improving emission. It is believed thatin this instance the acicular carbon particles in the electron fieldemitter material adhered to the tape protrude from the surface of theelectron field emitter material.

The electron field emitters with the improved emission propertiesprovided by this invention can be used in the cathodes of electronicdevices such as triodes and in particular in field emission displaydevices. Such a display device comprises (a) a cathode using an electronfield emitter that had undergone the process of the invention forimproving emission, (b) a patterned optically transparent electricallyconductive film serving as an anode and spaced apart from the cathode,(c) a phosphor layer capable of emitting light upon bombardment byelectrons emitted by the electron field emitter and positioned adjacentto the anode and between the anode and the cathode, and (d) one or moregate electrodes disposed between the phosphor layer and the cathode. Theuse of an adhesive material to improve the emission properties of anelectron field emitter is readily adapted to large size electron fieldemitters that can be used in the cathodes of large size display panels.

The process of this invention for improving the emission of an electronfield emitter is conducive to fabricating completely screen-printedtriodes. The electron field emitter can be subjected to the improvementprocess immediately after it is screen printed and fired or, preferably,after any dielectric materials and gate electrodes have been screenprinted onto the cathode and fired.

The accuracy and resolution attained with screen printing are limited.Therefore, it is difficult to fabricate a triode with dimensions lessthan 100 μm. Preventing electrical shorting between the gate and emitterlayers is difficult due to printing inaccuracy. In addition, since thefeatures on each layer must be printed one layer at a time, repeatedrepositioning of different screens further degrades registration. Inorder to prevent shorting, the gate layer opening is often enlargedrelative to the dielectric via and this significantly degrades theeffectiveness of the gate-switching field due to increased gate toemitter distance.

A photoimagable thick film approach can solve all of the aforementionedproblems and is useful for forming an array of normal gate triodes aswell as for forming an array of inverted-gate triodes. A normal gatetriode has the gate electrode physically between the field emittercathode and the anode. An inverted-gate triode has the field emittercathode physically between the gate and the anode. Photoimagable thickfilm formulations such as the Fodel® silver and dielectric pastecompositions (DC206 and DG201 respectively) are available from E. I. duPont de Nemours and Company, Wilmington, Del. They contain silver ordielectric in the form of fine particles and a small amount of lowmelting glass frit in an organic medium containing photoimagableingredients such as photoinitiator and photomonomers. Typically, auniform layer of Fodel® paste is screen printed on a substrate withcontrolled thickness. The layer is baked in low heat to dry. A contactphoto-mask with the desired pattern is placed in intimate contact withthe film and exposed to ultra-violet (UV) radiation. The film is thendeveloped in weak aqueous sodium carbonate. Feature dimensions as smallas 10 μm can be produce by photoimaging these screen-printed thickfilms. Therefore, triode dimensions as small as 25 μm may be achieved.In addition, imaging can be carried out in multi-layers thus eliminatingalignment accuracy problems. This is advantageous in the fabrication ofthe normal gate triode since the silver gate and dielectric layers canbe imaged together to achieve perfect alignment between the gate anddielectric openings and in the fabrication of the inverted gate triodesince the emitter, silver cathode, and dielectric layers can be imagedtogether to achieve perfect capping of the dielectric ribs whileavoiding short formation. For normal gate triodes with small viadimensions, it is preferred that the material providing the adhesiveforce in the process of the invention is applied in liquid form. Thisliquid adhesive is selected for a balance of adhesive, thermal andviscoelastic properties. A polymer solution or melt or a liquidpre-polymer containing thermal or ultra-violet curable polymer can beused.

The process of this invention for improving the emission of an electronfield emitter is also conducive to fabricating a lighting device. Such adevice comprises (a) a cathode using an electron field emitter that hasundergone the process of the invention for improving emission and (b) anoptically transparent electrically conductive film serving as an anodeand spaced apart from the cathode and (c) a phosphor layer capable ofemitting light upon bombardment by electrons emitted by the electronfield emitter and positioned adjacent to the anode and between the anodeand the cathode. The cathode may consist of an electron field emitter inthe form of a square, rectangle, circle, ellipse or any other desirableshape with the electron field emitter uniformly distributed within theshape or the electron field emitter may be patterned. Screen printing isa convenient method for forming the electron field emitter.

The presence of emission hot spots is a major disadvantage of anyapproach for forming the electron field emitter that involves having a“collection of random emitters”, e.g., using an emitter paste comprisedof acicular carbon such as nanotubes. To minimize hot spots, emitterpastes should be made as uniform as possible by using fine grainingredients and mixing and dispersion methods well known in the art.However, since it is ultimately not possible to control the exact aspectratio, orientation, and the local environment of each and every nanotubein a printed thick film surface, a natural statistical distribution ofturn-on voltages for the individual nanotube emitters is expected. Foruniform emission, this distribution should be as narrow as possible. Thenanotubes populating the low field side of this distribution will emitsignificantly higher current than the majority at a given field,resulting in emission hot spots. Hot spots clearly limit the achievableuniformity and contrast of a display. In addition, hot spots can alsoseverely limit the maximum dc anode voltage applied prior to the on-setof uncontrolled emission. This lower anode voltage in turn increases therequired switching gate voltage and reduces the display brightness dueto reduced phosphor efficiency. Therefore it is of great importance todiscover ways to selectively “quench” the hot spots without damaging thegeneral emission characteristic of the emitters. It has been found thatthe use of a reactive gas and gas plasma dramatically reduces hot spotemission and increases the achievable anode voltage prior to the on-setof uncontrolled emission. In addition, the hot spots were eliminatedwithout damage to general emission.

To selectively eliminate hot spots, a process must take advantage of thefact that hot spots consist of areas with abnormally high local emissioncurrent and electric field. This in turn creates local heating of thehot emitters and ionization of reactive gas in the immediate surroundingof the emitters. It is believed that the quenching process works byselectively reacting the carbon nanotubes at a hot spot with the highlyreactive gas and plasma surrounding the hot emitters without anydetrimental effect on the general emitter population that is notemitting electrons during the process. The self-terminating nature ofthis process is also consistent with this mechanism. When emissionsubsides, so do heating and plasma generation, thus terminating theprocess. Oxygen can be used as the gas. Other reactive gases and vapors,such as but not limited to ozone, hydrogen, halogens, hydrocarbon, andfluoro-chloro-carbons may also be effective.

Field emission tests were carried out on the resulting samples using aflat-plate emission measurement unit comprised of two electrodes, oneserving as the anode or collector and the other serving as the cathode.The cathode consists of a copper block mounted in apolytetrafluoroethylene (PTFE) holder. The copper block is recessed in a1 inch by 1 inch (2.5 cm ×2.5 cm) area of PTFE and the sample substrateis mounted to the copper block with electrical contact being madebetween the copper block and the sample substrate by means of coppertape. A high voltage lead is attached to the copper block. The anode isheld parallel to the sample at a distance, which can be varied, but oncechosen it was held fixed for a given set of measurements on a sample.Unless stated otherwise was a spacing of 1.25 mm was used. The anodeconsists of a glass plate coated with indium tin oxide deposited bychemical vapor deposition. It is then coated with a standard ZnS-basedphosphor, Phosphor P-31, Type 139 obtained from Electronic SpaceProducts International. An electrode is attached to the indium tin oxidecoating.

The test apparatus is inserted into a vacuum system, and the system wasevacuated to a base pressure below 1 ×10⁻⁵ torr (1.3×10⁻³ Pa). Anegative voltage pulse with typical pulse width of 3 μsec at a frequencyof 60 Hz is applied to the cathode and the emission current was measuredas a function of the applied voltage. The image emitted by the phosphoras a result of the emission current is recorded with a camera.

EXAMPLES OF THE INVENTION Example 1

This Example demonstrates the good emission exhibited by an electronfield emitter comprised of single wall carbon nanotubes after undergoingthe process of the invention for improving emission.

Laserablation grown single wall nanotubes were obtained from Tubes @Rice, Rice University, Houston, Tex. as a suspension in water. 20 ml ofthis suspension was diluted with 40 ml of distilled water and milled ona media mill for 2 hours. The resultant material was centrifuged for 2hours at 5000 rpm and the supernatant liquid removed. A sludge remainedwhich was found to contain 5% nanotube solids by a thermogravimetricanalysis determination. 1 gram of this material was added to 0.05 gramsof a glass frit, Bayer PK8701 (CAS Registry No. 65997-18-4) and 1.5grams of a typical organic medium composed primarily of ethylcellulosein terpineol. These ingredients were mixed on a glass plate muller for75 rotations to form the emitter paste. A pre-fired silvered glasssubstrate was prepared by screen printing a mixture of silver powder anda low melting glass frit in a typical organic ethylcellulose-basedmedium onto glass, followed by firing at 525° C. A 1 cm² square patternof emitter paste was then screen printed onto the pre-fired silveredglass substrate using a 325 mesh screen and the sample was subsequentlydried at 120° C. for 10 minutes. The dried sample was then fired innitrogen for 10 minutes at 450-525° C. After firing the nanotube/glasscomposite forms an adherent coating on the substrate. This electronfield emitter was tested for field emission as described in thespecification. After the emission test, a piece of Scotch™ Magic™ Tape,(#810 -3M Company) was applied to and contacted with the electron fieldemitter and then removed. A portion of the electron field emitteradhered to the Scotch™ Magic™ Tape. The electron field emitter was thentested for field emission. FIG. 1 shows the emission results for theelectron field emitter both as prepared and after undergoing the processof the invention for improving emission with emission current densityplotted as a function of applied electric field. The emission current isgreatly improved as a result of the process of the invention.

Example 2

This Example demonstrates the good emission exhibited by an electronfield emitter comprised of single wall carbon nanotubes after undergoingthe process of the invention for improving emission.

Carbolex AP-Grade single wall carbon nanotubes were obtained as a powderfrom Carbolex Inc., Lexington, Ky. 0.11 gram of this material was addedto 0.75 grams of a typical organic medium composed primarily ofethylcellulose in terpineol. These ingredients were mixed on a glassplate muller for 75 rotations to form the emitter paste. A pre-firedsilvered glass substrate was prepared by screen printing a mixture ofsilver powder and a low melting glass frit in a typical organicethylcellulose-based medium onto glass, followed by firing at 525° C. A1 cm² square pattern of emitter paste was then screen printed onto thepre-fired silvered glass substrate using a 325 mesh screen and thesample was subsequently dried at 120° C. for 10 minutes. The driedsample was then fired in nitrogen for 10 minutes at 450° C. After firingthe nanotube paste forms an adherent coating on the substrate. Thiselectron field emitter was tested for field emission as described in thespecification. After the emission test, a piece of Scotch™ Magic™ Tape,(#810-3M Company) was applied to and contacted with the electron fieldemitter and then removed. A portion of the electron field emitteradhered to the Scotch™ Magic™ Tape. The electron field emitter was thentested for field emission. FIG. 2 shows the emission results for theelectron field emitter both as prepared and after undergoing the processof the invention for improving emission with emission current densityplotted as a function of applied electric field. The emission current isgreatly improved as a result of the process of the invention.

Example 3

This Example demonstrates the good emission exhibited by an electronfield emitter comprised of catalytically grown carbon fibers afterundergoing the process of the invention for improving emission.

Catalytically grown carbon fibers were obtained as a powder fromCatalytic Materials Ltd, 12 Old Stable Drive, Mansfield, Mass. 0.1513grams of these catalytically grown carbon fibers were added to 0.1502grams of glass, Bayer PK 8701 (CAS Registry No. 65997-18-4), and 1.5012grams of a typical organic medium composed primarily of ethylcellulosein terpineol. These ingredients were mixed on a glass plate muller for75 rotations to form the emitter paste. A pre-fired silvered glasssubstrate was prepared by screen printing a mixture of silver powder anda low melting glass frit in a typical organic ethylcellulose-basedmedium onto glass, followed by firing at 525° C. A 1 cm² square patternof emitter paste was then screen printed onto the pre-fired silveredglass substrate using a 325 mesh screen and the sample was subsequentlydried at 120° C. for 10 minutes. The dried sample was then fired innitrogen for 10 minutes at 450° C. After firing the catalytically growncarbon fiber/glass composite forms an adherent coating on the substrate.This electron field emitter was tested for field emission as describedin the specification. After the emission test, a piece of Scotch™ Magic™Tape, (#810-3M Company) was applied to and contacted with the electronfield emitter and then removed. A portion of the electron field emitteradhered to the Scotch™ Magic™ Tape. The electron field emitter was thentested for field emission. FIG. 3 shows the emission results for theelectron field emitter both as prepared and after undergoing the processof the invention for improving emission with emission current plotted asa function of applied voltage. The emission current has improved by atleast three orders of magnitude, i.e., a factor of more than 1000, foreach of the voltages measured as a result of the process of thisinvention.

Example 4

This Example demonstrates the good emission exhibited by an electronfield emitter comprised of vapor grown carbon fibers after undergoingthe process of the invention for improving emission.

Vapor grown carbon fibers were obtained as a powder from Showa DenkoAmerica, San Mateo, Calif. 0.11 gram of this material was added to 0.75grams of a typical organic medium composed primarily of ethylcellulosein terpineol. These ingredients were mixed on a glass plate muller for75 rotations to form the emitter paste. A pre-fired silvered glasssubstrate was prepared by screen printing a mixture of silver powder anda low melting glass frit in a typical organic ethylcellulose-basedmedium onto glass, followed by firing at 525° C. A 1 cm² square patternof emitter paste was then screen printed onto the pre-fired silveredglass substrate using a 325 mesh screen and the sample was subsequentlydried at 120° C. for 10 minutes. The dried sample was then fired innitrogen for 10 minutes at 450° C. After firing the vapor grown carbonfiber paste forms an adherent coating on the substrate. This electronfield emitter was tested for field emission as described in thespecification. After the emission test, a piece of Scotch™ Magic™ Tape,(#810-3M Company) was applied to and contacted with the electron fieldemitter and then removed. A portion of the electron field emitteradhered to the Scotch™ Magic™ Tape. The electron field emitter was thentested for field emission. FIG. 4 shows the emission results for theelectron field emitter both as prepared and after undergoing the processof the invention for improving emission with emission current densityplotted as a function of applied electric field. The emission current isgreatly improved as a result of the process of the invention.

Examples 5-8

These Examples demonstrate the use of carbon nanotube/silver emitterpaste with low concentrations of single wall carbon nanotubes to screenprint electron field emitters and the good emission exhibited by theseelectron field emitters after undergoing the process of the inventionfor improving emission.

The emitter pastes for Examples 5-8 were prepared by mixing twocomponents, one a paste containing single wall carbon nanotubes and theother a silver paste. Laser ablation grown single wall carbon nanotubeswere obtained from Tubes @ Rice, Rice University, Houston, Tex. as asuspension in toluene (7.7 grams nanotubes per liter of toluene). Aportion of this suspension was mixed with the typical organic mediumcomposed primarily of ethylcellulose in terpineol to form a nanotubepaste. The quantity of nanotubes in this nanotube paste was 0.29 wt %.The silver paste was a silver paste composition 7095 available from E.I. du Pont de Nemours and Company, Wilmington, Del. containing 65.2 wt %silver in the form of fine silver particles and a small amount of glassfrit in an organic medium. The emitter pastes for Examples 5-8 wereprepared by combining the nanotube/silver pastes in the ratios by weightof 80/20, 60/40, 40/60 and 20/80 respectively. Each of thesecombinations were mixed on a glass plate muller for 75 rotations to formthe emitter paste. A pre-fired silvered glass substrate was prepared foreach Example by screen printing a mixture of silver powder and a lowmelting glass frit in a typical organic ethylcellulose-based medium ontoglass, followed by firing at 525° C. For each Example, a 9/16 inch (1.43cm) square pattern of emitter paste was then screen printed onto thepre-fired silvered glass substrate using a 325 mesh screen and thesample was subsequently dried at 120° C. for 10 minutes. The driedsample was then fired in nitrogen for 10 minutes at 450° C. After firingthe nanotube/silver composite forms an adherent coating on thesubstrate. These fired electron field emitters of Examples 5-8respectively contained 3.49 wt %, 1.34 wt %, 0.60 wt % and 0.23 wt %nanotubes in a silver matrix where the weight percentages werecalculated neglecting the small amount of glass present. The actualweight percentages of the nanotubes on the basis of the total weight ofthe electron field emitter would therefore be slightly lower. Eachelectron field emitter was then tested for electron field emission asdescribed previously. Only discrete emission sites were observed and thetotal emission current was low for each Example even at high electricfields. After the emission test, a piece of Scotch™ Magic™ Tape,(#810-3M Company) was applied to and contacted with the electron fieldemitter of each Example and then removed. A portion of each electronfield emitter adhered to the Scotch™ Magic™ Tape. The electron fieldemitter of each Example was then tested for field emission and eachshowed a uniform and high density emission across the entire surface ofthe electron field emitter. The currents observed for the electron fieldemitters of Examples 5 and 6 with applied voltages of 2 kV were 30 and27 μA respectively and those observed for the electron field emitters ofExamples 7 and 8 with applied voltages of 2.5 kV were 17 and 15 μArespectively, all orders of magnitude higher in current than observedbefore the electron field emitters underwent the process of theinvention for improving emission.

Examples 9-11

These Examples demonstrate the use of carbon nanotube/dielectric emitterpaste with low concentrations of single wall carbon nanotubes to screenprint electron field emitters and the good emission exhibited by theseelectron field emitters after undergoing the process of the inventionfor improving emission.

The emitter pastes for Examples 9-11 were prepared by mixing twocomponents, one a paste containing single wall carbon nanotubes and theother a paste containing a dielectric. Laser ablation grown single wallcarbon nanotubes were obtained from Tubes @ Rice, Rice University,Houston, Tex. as a suspension in toluene (7.7 grams nanotubes per literof toluene). A nanotube paste was prepared by mixing two portions byweight of this suspension with one portion by weight of ethylcellulosebinder in terpineol. The dielectric paste was prepared from a mixture ofa low softening bismuth borate frit, alumina filler, ethylcellulosebinder, <1% of blue pigment, <1% of a phospate surfactant, andterpineol.

The emitter pastes for Examples 9-11 were prepared by combining thenanotube/dielectric pastes in the ratios by weight of 2/3, 1.2/1.55 and1/4 respectively. Each of these combinations were mixed on a glass platemuller for 75 rotations to form the emitter paste. A pre-fired silveredglass substrate was prepared for each Example by screen printing amixture of silver powder and a low melting glass frit in a typicalorganic ethylcellulose-based medium onto glass, followed by firing at525° C. For each Example, a 9/16 inch (1.43 cm) square pattern ofemitter paste was then screen printed onto the pre-fired silvered glasssubstrate using a 325 mesh screen and the sample was subsequently driedat 120° C. for 10 minutes. The dried sample was then fired in nitrogenfor 10 minutes at 450° C. After firing the nanotube/dielectric compositeforms an adherent coating on the substrate. These fired electron fieldemitters of Examples 9-11 respectively contained 0.47 wt %, 0.91 wt %,and about 0.07 wt % nanotubes in the dielectric matrix where the weightpercentages were calculated on the basis of the total weight of theelectron field emitter. Each electron field emitter was then tested forelectron field emission as described previously. Only discrete emissionsites were observed. After the emission test, a piece of Scotch™ Magic™Tape, (#810-3M Company) was applied to and contacted with the electronfield emitter of each Example and then removed. A portion of eachelectron field emitter adhered to the Scotch™ Magic™ Tape. The electronfield emitter of each Example was then tested for field emission andeach showed a uniform and high density emission across the entiresurface of the electron field emitter. FIG. 5 shows the emission resultsfor the electron field emitter both as prepared and after undergoing theprocess of the invention for improving emission for each of the threeExamples with emission current density plotted as a function of appliedelectric field. FIGS. 6 a and 6 b are photographs of the light emittedfrom a phosphor layer impinged by electron emission from the electronfield emitter of Example 9 before and after undergoing the process ofthe invention for improving emission. FIGS. 6 c and 6 d are photographsof the light emitted from a phosphor layer impinged by electron emissionfrom the electron field emitter of Example 10 before and afterundergoing the process of the invention for improving emission. FIGS. 6e and 6 f are photographs of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitter of Example11 before and after undergoing the process of the invention forimproving emission. The emission current is greatly improved as a resultof the process of the invention for the electron field emitters of allthree Examples.

Example 12

This Example demonstrates the good emission exhibited by an electronfield emitter comprised of single wall carbon nanotubes after undergoingthe process of the invention for improving emission.

Carbolex AP-Grade single wall carbon nanotubes were obtained as a powderfrom Carbolex Inc., Lexington, Ky. 0.11 gram of this material was addedto 0.75 rams of a typical organic medium composed primarily ofethylcellulose in terpineol. These ingredients were mixed on a glassplate muller for 75 otations to form the emitter paste. Two essentiallyidentical screen-printed emitter paste samples were prepared. Twopre-fired silvered glass substrates were prepared by screen printing amixture of silver powder and a low melting glass frit in a typicalorganic ethylcellulose-based medium onto two pieces of glass, followedby firing at 525° C. A 1 cm² square pattern of emitter paste was thenscreen printed onto each of the pre-fired silvered glass substratesusing a 325 mesh screen and the samples were subsequently dried at 120°C. for 10 minutes. A sandwich structure of substrate-emitterpaste-substrate was formed by contacting the two emitter paste samplesand lightly pressing together the two substrates. The sandwich structurewas then fired in nitrogen for 10 minutes at 450° C. The two substrateswere then separated from one another by pulling them apart therebyfracturing the fired emitter paste and providing two electron fieldemitters each having a newly formed surface. Each electron field emitterwas tested for emission as described previously. FIG. 7 shows theemission results for both electron field emitters with emission currentdensity plotted as a function of applied electric field. Both electronfield emitters show higher emission than an electron field emitter whichhas been fired but not subjected to other treatment, the results forwhich are shown in FIG. 2. In this Example no adhesive tape wasrequired. The improvement is achieved during the separation of thesubstrates following firing and the resulting fracture of the firedemitter paste.

Example 13-14

These Examples demonstrates the use of a thermally softened polymer filmas the material to provide the adhesive force with the electron fieldemitter.

The emitter paste for these Examples was prepared by mixing threecomponents: one a suspension containing single wall carbon nanotubes,one a typical organic medium containing 10% ethylcellulose and 90%beta-terpineol, and one a typical paste containing silver. Laserablation grown single wall carbon nanotubes were obtained from Tubes @Rice, Rice University, Houston, Tex. as an unpurified powder produced bylaser ablation. A nanotube suspension was prepared by sonicating, i.e.by mixing ultrasonically, a mixture containing 1% by weight of thenanotube powder and 99% by weight of trimethylbenzene. The ultrasonicmixer used was a Dukane Model 92196 with a ¼ inch horn operating at 40kHz and 20 watts. The silver paste was a silver paste composition 7095available from E. I. du Pont de Nemours and Company, Wilmington, Del.containing 65.2 wt % silver in the form of fine silver particles and asmall amount of glass frit in an organic medium.

The emitter paste was prepared by combining the nanotubesuspension/organic medium/silver pastes in the ratios by weight of27/40/33. The combination was mixed in a three-roll mill for ten passesto form the emitter paste. A pre-fired silvered glass substrate wasprepared for each Example by screen printing a mixture of silver powderand a low melting glass frit in a typical organic ethylcellulose-basedmedium onto glass, followed by firing at 525° C. For Example 13, a 9/16inch (1.43 cm) square pattern of emitter paste was screen printed ontothe pre-fired silvered glass substrates using a 325 mesh screen. ForExample 14, an 11×11 pixel array (with pixel diameters of 20 mil sizeand spaced so that the distance between neighboring pixels was 40 mil.These two samples were subsequently dried at 120° C. for 10 minutes. Thesamples were then fired in nitrogen for 10 minutes at 525° C. Afterfiring, the nanotube/silver composite forms an adherent coating on thesubstrate. These fired electron field emitters contained 1.3 wt % ofnanotubes in a mostly silver matrix where the weight percentage wascalculated on the basis of the total weight of the electron fieldemitter after firing. The electron field emitter samples were thentested for electron field emission as described previously. Rather thanuniform emission from the square pattern of Example 13 or over the wholeemitter surface of each pixel of Example 14, only discrete emissionsites were observed in the as-fired samples.

A low melting Riston® polymer film available from E. I du Pont deNemours and Company was used as the polymer film which was thermallysoftened and provided the adhesive contact with the electron fieldemitter. As shown in FIG. 8 a, for easy handling the low melting acrylicpolymer film 1 of 3 mil thickness was either heat extruded onto a 1 milthick polyethylene terephthalate (PET) backing 2. A 1 mil polyethylenefilm 3 with a matted surface texture was used as a cover layer to form athree layer structure.

The same procedure was used for Examples 13 and 14. The polyethylenecover layer 3 was first removed exposing the acrylic polymer 1 as shownin FIG. 8 b. At room temperature the acrylic polymer was tack free andcan be easily placed on the emitter sample surface in preparation forheat lamination. The matted textured polymer surface facilitates airescape during heat lamination. If necessary, a vacuum bag or table canbe used to remove all air between the polymer film and sample surfaceprior to heat lamination thus insuring good contact to surfacetopography. The polymer film 1 with the PET backing 2 and the sample ofExample 14 with the silvered glass substrate 4 and the 11×11 pixel arrayof electron field emitter 5 were passed through a home/office laminatorheated to 60-70° C. with pressure adjusted for bubble free lamination toproduce the heat laminated polymer on the emitter as shown in FIG. 8 c.Although not carried out in this example, the PET backing 2 should bepolymer coated or plasma treated to provide for maximum adhesion to thelow melting polymer. For the untreated PET backing used in theseExamples, adhesion of the acrylic polymer to the PET was poor thusrequiring two additional steps. First the laminated sample was allowedto cool to room temperature and the PET backing 2 was removed as shownin FIG. 8 d. Then, a 1 mil thick aluminum foil 6, which adhered well tothe acrylic polymer, was heat laminated onto the acrylic polymer 1 asshown in FIG. 8 e. These two steps can be eliminated when a coated ortreated PET backing is used. Upon cooling to room temperature, thealuminum backed polymer film was peeled off the emitter surface leavingthe emitter shown in FIG. 8 f. A portion of the electron field emitterof each sample adhered to the acrylic polymer.

The electron field emitters of Examples 13 and 14 were then tested forfield emission as previously described except that the spacing used forExample 13 was 1.85 mm. There was a uniform and high density emissionacross the entire surfaces of the patterned surface of the electronfield emitter of Example 13 and of the large square of electron fieldemitter of Example 14. The current observed for Example 13 with anapplied voltage of 2 kV was 6 μA. The current observed for Example 14with an applied voltage of 3 kV was 80 μA.

FIG. 9 a is a photograph of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitter of Example13 after undergoing the process of the invention for improving emissionand FIG. 9 b is a photograph of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitter of Example14 after undergoing the process of the invention for improving emission.

Example 15

This Example demonstrates the use of a photoimagable carbonnanotube/silver emitter paste with low concentrations of single wallnanotubes to screen print and photo-image a pattern of electron fieldemitters and the good emission exhibited by these electron fieldemitters after undergoing the activation process of the invention forimproving emission.

The emitter paste for Example 15 was prepared by mixing two components,one a powder containing single wall carbon nanotubes and the other aphotoimagable paste containing silver. Laser ablation grown single wallcarbon nanotubes were obtained from Tubes @ Rice, Rice University,Houston, Tex. as an unpurified powder produced by laser ablation. Thesilver paste composition is DC206 available from E. I. du Pont deNemours and Company, Wilmington, Del. It contains silver in the form offine silver particles and a small amount of glass frit in an organicmedium containing photoimagable ingredients such as photoinitiator andphotomonomers.

The photoimagable emitter paste was prepared by combining the carbonnanotube powder and the Fodel® silver paste in the ratios by weight of1/100 on a glass plate muller and mixing for 75 rotations. A pre-firedsilvered glass substrate was prepared by screen printing a mixture ofsilver powder and a low melting glass frit in a typical organicethylcellulose-based medium onto glass, followed by firing at 525° C. A⅞ inch (2.22 cm) square pattern of photoimagable emitter paste was thenscreen printed under yellow light onto the pre-fired silvered glasssubstrate using a 325 mesh screen and the sample was subsequently driedat 120° C. for 10 minutes. The dried sample was then photo-patterned byusing a photo tool containing an UV light transparent pattern of“DUPONT”. An UV dose of 4000 mJ was used for exposure. The exposedsample was developed in 0.5% carbonate aqueous solution to wash out theunexposed area of the sample. The developed sample was then rinsedthoroughly in water and allowed to dry. The sample was then fired innitrogen for 10 minutes at 525° C. in nitrogen. After firing, thenanotube/Fodel® silver composite forms an adherent coating on thesubstrate. This electron field emitter was then tested for electronfield emission as described previously. Only discrete emission siteswere observed. After the emission test, a piece of Scotch™ Magic™ Tape,(#810-3M Company) was applied to and contacted with the electron fieldemitter of then removed. A portion of the electron field emitter adheredto the Scotch™ Magic™ Tape. The electron field emitter was then testedfor field emission and showed a uniform and high density emission acrossthe entire patterned surface of the electron field emitter.

FIG. 10 is a photograph of the light emitted from a phosphor layerimpinged by electron emission from the electron field emitter afterundergoing the process of the invention for improving emission.

Example 16

This Example demonstrates the use of silver, dielectric, and carbonnanotube/silver emitter pastes in the construction of a fullyscreen-printed electron field emissive triode array with the carbonnanotube electron field emitter deposited within the vias of the triodestructure and the good emission obtained after undergoing the process ofthe invention for improving emission.

The silver paste was silver paste composition 7095, available from E. I.du Pont de Nemours and Company, Wilmington, Del., containing 65.2 wt %silver in the form of fine silver particles and a small amount of glassfrit in an organic medium. The dielectric paste was prepared from amixture of a low softening bismuth borate frit, alumina filler,ethylcellulose binder, <1% of blue pigment, <1% of a phosphatesurfactant, and terpineol.

Laser ablation grown single wall nanotubes were obtained from Tubes @Rice, Rice University, Houston, Tex. as an unpurified power produced bylaser ablation. The emitter paste was prepared by combining 0.03 gramsof nanotube powder with 2.97 grams of 7095 silver paste with a muller toproduce the emitter paste with 1.0 wt % nanotubes in the silver paste.

The layers forming the triode are shown in FIG. 11 with the separatelayers indicated in FIG. 11 a and the finished triode structure shown inFIG. 11 b. A silver cathode line pattern 6 μm thick was prepared on aglass substrate 1 by screen printing 7095 silver paste in a five-linepattern 2 using a 325 mesh screen, each line being 30 mils wide, andthen firing for ten minutes at 525° C. Two layers of dielectric 3 werethen screen printed on top of the cathode lines with a 280 mesh screencontaining a 5×5 array of vias, each via having a diameter of 20 milsand being centered on the silver cathode lines. The total thickness ofthe dielectric was 25 μm. The dielectric was then fired in air at 500°C. A 6 μm single layer of the same 7095 silver paste was then screenprinted on top of the dielectric using a 325 mesh screen, in five linesperpendicular to the cathode lines, to function as the gate electrode 4.The gate has vias 28 mil in diameter with centers corresponding to thecenters of the vias in the dielectric. The gate layer was then fired inair at 500° C. during which the via diameter was reduced to 25 mils.After firing, the silver layers were 5 μm thick and the dielectric layerwas 25 μm thick.

Finally, vias were filled with the emitter paste 5 using a via fillscreen with 20 mil holes and then fired to 450° C. in N₂ to protect thenanotubes from oxidation. The fired emitter paste plug diameter is 19mils at the top. The resulting device had a gate-to-via distance of 3mils.

The as-fired triode array was first tested for electron emission in thediode mode as described above. The gate electrode was allowed to floatelectrically during the diode testing. No emission was observed from theas-fired sample at an anode-cathode voltage of 4 kV. The triode arraywas then subjected to the process of the invention by contacting thetriode surface with a piece of Scotch™ Magic™ Tape, (#810-3M Company).The tape was pressed onto the surface and into the via openings with asoft rubber roller so that the tape would make contact with the electronemitters. Vacuum may be applied to eliminate trapped air inside the viaopening to obtain optimal adhesive contact with the emitter. The tapewas then peeled off. A portion of the electron field emitter adhered tothe Scotch™ Magic™ Tape and could be seen as dark spots on the tape. Thetriode was again tested for diode emission in the apparatus describedpreviously. The light generated on the phosphor anode during the diodetest of the screen-printed triode array is shown in FIG. 12. Emissionwas observed from all 25 vias as shown in FIG. 12 a. An emission currentof 130 nA was measured at an anode-cathode voltage of 3.25 kV.

Using the same test apparatus, the triode array was tested in the triodemode by connecting 4 of the 5 gate lines to a pulsed gate power supply,all cathode lines to a DC cathode power supply, and the anode to a DCanode power supply. The one unconnected gate line was left floatingelectrically, thus providing a control for diode versus triode emission.With the gate power supply set at ground, the anode power supply was setto 3 kV and the cathode power supply to −300V. Weak diode emissions wereobserved at these voltage settings from the vias associated with thefloating gate line, i.e., the 2nd line from the left in FIG. 12 b. Thegate power supply was then set to produce a pulsed voltage of 100 V at60 Hz with 3 μsec pulse duration. Strong triode emission was observedfor all vias associated with gate lines driven by the gate power supply,i.e., the 1 st, 3rd, 4th, and 5th lines from the left in FIG. 12 b. Atriode emission current of 600 nA was measured at the anode.

Example 17

This Example demonstrates the use of a carbon nanotube/silver emitterpaste with low concentrations of single wall nanotubes to screen printelectron field emitters for use in a lighting device. Carbonnanotube/silver emitter paste with low concentrations of single wallnanotubes was used to screen print electron field emitters. Afterundergoing the process of the invention for improving emission, theseelectron field emitters exhibited strong and uniform emission. Furthermore, the emitters were driven with high anode voltage and to high dutycycles in order to achieve high brightness suitable for lightingapplications.

The emitter paste for Example 17 was prepared by mixing single wallcarbon nanotube powder into a paste containing silver. Laser ablationgrown single wall carbon nanotubes were obtained from Tubes @ Rice, RiceUniversity, Houston, Tex. as a unpurified power produced by laserablation. The silver paste was silver paste composition 7095 availablefrom E. I. du Pont de Nemours and Company, Wilmington, Del. containing65.2 wt % silver in the form of fine silver particles and a small amountof glass frit in an organic medium. The emitter paste was prepared bycombining 0.03 grams of carbon nanotube powder with 2.97 grams of 7095silver paste with a muller to give an emitter paste containing 1.0 wt %nanotubes. Pre-fired silvered glass substrates were prepared by screenprinting a mixture of 7095 silver paste onto the glass substrate,followed by firing at 525° C. for 10 minutes in a belt furnace. A 9/16inch (1.43 cm) square pattern of emitter paste was then screen printedonto the pre-fired silvered glass substrate using a 325 mesh screen andthe sample was subsequently dried at 120° C. for 10 minutes. The sampleswere then fired in nitrogen for 10 minutes at 450° C. After firing, thenanotube/silver composite forms an adherent coating on the substrate.The fired electron field emitter contained 1.4 wt % of nanotubes in amostly silver matrix where the weight percentage was calculated on thebasis of the total weight of the carbon nanotube powder and thenon-volatile solids of the 7095 paste.

The electron field emitter sample was then tested for electron fieldemission as described previously. Only discrete emission sites wereobserved in the as-fired samples. After this first emission test, apiece of Scotch™ Magic™ Tape, (#810-3M Company) was applied to andcontacted with the electron field emitter and then removed. A portion ofthe electron field emitter adhered to the Scotch™ Magic™ Tape. Theelectron field emitter was then tested for field emission and showed auniform and high density emission across the entire surface of theelectron field emitter.

In order to demonstrate the potential of these electron emitters inlighting related applications, the anode-cathode spacing was increasedto 3 mm, which allowed for the use of an anode voltage of 6 kV whilemaintaining an electric field of 2 V/μm on the emitters. The experimentwas carried out by biasing the anode at a constant voltage of 3 kV.Negative 3 kV voltage pulses with 60 Hz repetition rate was applied tothe cathode. The pulse duration was allowed to vary from 3 μsec to 3msec thus spanning a range of 0.018% to 18% in duty cycle. The emissioncurrent density and the phosphor luminance output was found to increasewith duty cycle, reaching values of 190 μA/cm² and 12000 Cd/m²respectively. This luminance value is twice that of a typicalfluorescent lamp. The energy efficiency, which was not optimized, wasfound to be about 30% that of a fluorescent lamp and 150% that of anincandescent lamp.

Examples 18-22

These Examples demonstrate the emission performance of electron fieldemitters made with carbon nanotubes obtained from five different sourcesafter undergoing the process of the invention for improving emission.

An emitter paste was prepared for each Example by mixing threecomponents, one a suspension containing carbon nanotubes, one a typicalorganic medium containing 10% ethylcellulose and 90% beta-terpineol andone a typical paste containing silver. A nanotube suspension wasprepared by sonicating a mixture containing about 1% by weight of thenanotubes and 99% by weight of terpineol. The nanotubes used in eachExample are:

-   -   Example 18—Laser ablation grown single wall nanotubes from Tubes        @ Rice, Houston, Tex.    -   Example 19—Hipco process nanotubes from Carbon Nanotechnologies,        Inc., Houston, Tex.    -   Example 20—Single wall nanotubes from MER Inc., Tuscon, Ariz.    -   Example 21—Carbolex AP-Grade single wall nanotubes from Carbolex        Inc., Lexington, Ky.    -   Example 22—Multiwall nanotubes from Nanolab Inc., Watertown,        Mass. The silver paste was the silver paste composition 7095,        available from E. I. du Pont de Nemours and Company, Wilmington,        Del., described previously.

The emitter paste was prepared by combining the nanotubesuspension/organic medium/silver pastes in the ratios by weight of about30/40/30 on a three-roll mill for 10 passes. A pre-fired silvered glasssubstrate was prepared for each Example by screen printing thepreviously described 7095 silver paste onto glass, followed by firing at525° C. A 9/16 inch (1.43 cm) square, uniform pattern of emitter pastewas then screen printed onto the pre-fired silvered glass substrateusing a 325 mesh screen and the samples were subsequently dried at 120°C. for 10 minutes. All the samples were then fired in nitrogen for 10minutes at 525° C. After firing the nanotube/silver composite forms anadherent coating on the substrate. The fired electron field emitters ofthe Examples each contain about 1 wt % of nanotubes in a mostly silvermatrix where the weight percentages were calculated on the basis of thetotal weight of the electron field emitter.

A piece of Scotch™ Magic™ Tape, (#810-3M Company) was applied to andcontacted with the electron field emitter of each Example and thenremoved. A portion of each electron field emitter adhered to the Scotch™Magic™ Tape. The electron field emitter of each Example was then testedfor field emission as described previously. Table 1 compares theemission results for the electron field emitters after undergoing theprocess of the invention for improving emission with emission currentdensity given as a function of applied electric field. It is clear thatthe nanotubes used in Example 18 exhibit the highest current density ofany of those use and are therefore most preferred.

TABLE 1 Applied Electric Current Density (A/cm²) Voltage Field ExampleExample Example Example Example (kV) (V/μm) 18 19 20 21 22 0.5 nm 0.4 nmnm nm nm 1.0 0.8 1.75 × 10⁻⁸  8.5 × 10⁻¹⁰ nm nm nm 1.5 1.2 5.50 × 10⁻⁷1.50 × 10⁻⁸  4.00 × 10⁻¹⁰ 2.50 × 10⁻⁹ nm 2.0 1.6 5.60 × 10⁻⁶ 7.50 × 10⁻⁸7.50 × 10⁻⁹ 4.50 × 10⁻⁸  5.00 × 10⁻¹¹ 2.5 2.0 1.90 × 10⁻⁵ 2.50 × 10⁻⁷5.50 × 10⁻⁸ 2.50 × 10⁻⁷  3.50 × 10⁻¹⁰ 3.0 2.4 4.45 × 10⁻⁵ 7.00 × 10⁻⁷3.00 × 10⁻⁷ 1.15 × 10⁻⁶ 1.05 × 10⁻⁹ 3.5 2.8 1.45 × 10⁻⁶ 9.00 × 10⁻⁷ 2.75× 10⁻⁶ 5.00 × 10⁻⁹ 4.0 3.2 3.00 × 10⁻⁶ 2.25 × 10⁻⁶ 6.50 × 10⁻⁶ 1.00 ×10⁻⁸ nm = not measurable, i.e., less than 1 × 10⁻¹¹ A/cm²

Example 23

This Example demonstrates the use of oxygen as a reactive gas and gasplasma to dramatically reduce hot spot emission and increase theachievable anode voltage prior to the on-set of uncontrolled emission.It further demonstrates that hot spots can be eliminated without damageto general emission.

The emitter paste for this Example was prepared by mixing threecomponents: one a suspension containing single wall carbon nanotubes,one a typical organic medium containing 10% ethylcellulose and 90%beta-terpineol, and one a typical paste containing silver. Laserablation grown single wall carbon nanotubes were obtained from Tubes @Rice, Rice University, Houston, Tex. as an unpurified powder. A nanotubesuspension was prepared by sonicating, i.e. by mixing ultrasonically, amixture containing about 1% by weight of the nanotube powder and 99% byweight of dibutyl carbitol. The ultrasonic mixer used was a Dukane Model92196 with a ¼ inch horn operating at 40 kHz and 20 watts. The silverpaste was the silver paste composition 7095, available from E. I. duPont de Nemours and Company, Wilmington, Del., described previously.

The emitter paste was prepared by combining the nanotubesuspension/organic medium/silver pastes in the ratios by weight of about30/40/30. The combination was mixed in a three-roll mill for ten passesto form the emitter paste. A pre-fired silvered glass substrate wasprepared by screen printing the previously described 7095 silver pasteonto glass, followed by firing at 525° C. A 9/16 inch (1.43 cm) squarepattern of emitter paste was screen printed onto the pre-fired silveredglass substrates using a 325 mesh screen. The sample was subsequentlydried at 120° C. for 10 minutes. The sample was then fired in nitrogenfor 10 minutes at 525° C. After firing, the nanotube/silver compositeforms an adherent coating on the substrate.

A piece of Scotch™ Magic™ Tape, (#810-3M Company) was applied to andcontacted with the electron field emitter and then removed. A portion ofthe electron field emitter adhered to the Scotch™ Magic™ Tape. Theelectron field emitter was then tested for field emission.

The sample was then placed in the flat-plate emission measurement unitdescribed previously with an anode to cathode spacing of 1.25 mm. Anegative pulsed high voltage power supply was connected to the cathodeand thereby to the electron field emitter. The flat-plate emissionmeasurement unit was placed inside a vacuum chamber equipped with argonand oxygen gas inlets and flow controls. The vacuum chamber wasevacuated to a pressure equal to or below 1.0×10⁻⁵ torr (1.3×10⁻³ Pa).In order to establish the presence of emission hot spots on the sample,the anode voltage was gradually raised to 1 kV dc. Emission current andluminance of the phosphor screen were observed at an anode voltage aslow as 700 V. The hot spot emission current, measured at the anode powersupply was found to be 15 μA at 1 kV. More than a thousand strongdiscrete emission spots were observed in the 2 cm² sample. For a displaydevice operating at 1 kV anode voltage, all hot spots must beeliminated. At a higher anode voltage of 1.3 kV, the number of hot spotsand the total current increased to several thousands and 50 μArespectively. While maintaining the hot spot emission at 1.3 kV, argongas was introduced into the vacuum chamber through a flow control valveto raise the chamber pressure from 1.0×10⁻⁵ torr (1.3×10⁻³ Pa) to5.5×10⁻⁴ torr (7.3×10⁻² Pa). This was done to determine the effect ofchamber pressure on hot spot emission. No significant effect on thenumber of hot spots or the total emission current was observed with thechemically inert argon gas. After pumping out the argon gas, oxygen gaswas introduced into the chamber to a pressure of 5.5×10⁻⁴ torr (7.3×10⁻²Pa). Dramatic reductions in the number of hot spots and the totalemission current were observed within a few seconds. The visible hotspots on the sample dropped from several thousand to a small numberwithin ten minutes of the introduction of oxygen. Correspondingly, thetotal emission current dropped from 50 to less than 2 μA. However, therate of hot spot emission reduction was found to decrease with theemission current and the process appeared to self-terminate. A slightrecovery of the hot spot emission was also observed immediately afterthe oxygen was evacuated from the vacuum chamber. This residual currentstabilized at about 7 μA after 20 minutes.

Due to the self-terminating nature and the slight emission recovery, theprocess for reducing the hot spot emission described in this exampleshould be carried out at an anode voltage higher than the final deviceanode voltage. No hot spot or emission current was observed when theanode voltage was reduced to 1 kV. Therefore, this example demonstratesthe total elimination of over one thousand hot spots observed at 1 kVprior to oxygen quenching of this emission. With the anode held at 1 kV,a negative pulsed voltage of 1.5 kV with pulse duration of 3 μsec andrepetition rate of 60 Hz was applied to the cathode. High density anduniform emission was observed with a total emission current measured at25 μA This demonstrates that no significant damage to the generalemission occurred during the oxygen quenching process.

Example 24

The use of photoimagable silver, dielectric, and nanotube/silver emitterpastes in the construction of a thick film based field emissive triodearray results in feature size and alignment accuracy superior to thoseachievable with screen printing alone and the use of the process of theinvention for improving emission of such a triode can be achieved asdescribed in this Example.

A normal gate field emission triode has the gate electrode physicallybetween the electron field emitter cathode and the anode. Herein, thegate electrode is considered part of the cathode assembly. The cathodeassembly consists of a cathode current feed as a first layer depositedon the surface of a substrate. A dielectric layer, containing circularor slot shaped vias, forms the second layer of the device. An electronfield emitter layer is in contact with the cathode conductor within thevias and its thickness may extend from the base to the top of thedielectric layer. A gate electrode layer, deposited on the dielectricbut not in contact with the electron field emitters, forms the top layerof the cathode assembly. Critical dimensions in the cathode includes thevia diameter, the dielectric thickness, and the gate to electron fieldemitter distance. All these dimensions must be minimized to achieveoptimized low voltage switching of the triode.

The following process will result in the fabrication of a cathodeassembly for a normal gate triode array using photoimagable thick films.Various modifications in the steps will be obvious to those skilled inthe art. The process for making a cathode assembly with a normal gatetriode array using photoimagable thick film pastes comprises:

-   -   (a) printing on a substrate a photoimagable silver cathode        layer, photo-imaging and developing the silver cathode layer and        then firing to produce silver cathode feed lines on the        substrate,    -   (b) printing a photoimagable electron field emitter layer on top        of the silver cathode feed lines and the exposed substrate,        photoimaging and developing the electron field emitter layer        into dots, rectangles, or lines on the silver cathode feed        lines,    -   (c) printing one or more uniform photoimagable dielectric layers        on top of the silver cathode feed lines and the electron field        emitters and drying the dielectric,    -   (d) printing a layer of photoimagable silver gate lines on top        of the dielectric and drying this layer of silver gate lines,    -   (e) using a photo-mask containing the via or slot pattern to        image both the silver gate and the dielectric layers in a single        exposure, thereby placing the vias directly on top of the        electron field emitter dots, rectangles, or lines, and    -   (f) developing the silver gate and dielectric layers to reveal        the electron field emitter layer at the base of the vias and        co-firing the electron field emitter, dielectric, and silver        gate layers under conditions that are compatible with the        electron field emitter.

The cathode assembly containing the triode array is then ready toundergo the process of the invention for improving emission.

In step (b) of the process for making a cathode assembly with a normalgate triode array, the alignment of the subsequent dielectric and gatelayers can be simplified if the size of the dots, rectangles, or linesof the electron field emitter layer are significantly larger than thefinal via dimension. Alternatively, this electron field emitter layermay be fabricated by simple screen printing if this can be accomplishedfor the desired pitch density of the array and will not require the useof a photoimagable emitter paste. In step(d), if the pitch density istoo high for the printing of silver gate lines, a uniform layer ofphotoimagable silver can be printed and the lines subsequently formed inthe imaging step (e) using a mask with a silver gate line and viapattern.

The above process demonstrates how perfect registration of the gate,via, and electron field emitter can be achieved without any criticalalignment step when photoimagable thick films are used. Mostimportantly, this process prevents the formation of shorts between thegate and electron field emitter layers while at the same time achievingminimum gate to emitter separation. This process is applicable to allphoto-definable materials in addition to thick films. As an example, a50 μm diameter via array with 100 μm pitch was fabricated by co-imagingFodel® silver and dielectric layers. The array was fabricated from 18 μmdried Fodel® DG201-type dielectric with 13 μm of Fodel® silver on top.The array was imaged with 100 mJ, of UV light and developed at 1.5×TTC(standard development) in aqueous alkaline. It was then fired in a5-zone furnace with about a 10 minute peak temperature of 575° C.

To practice the process of the invention for improving emission on theelectron field emitter material in the above described cathode assemblyarray, a layer of said liquid adhesive is coated on the cathode assemblyby screen-printing or some other coating technique well known in theart. The adhesive material is allowed to dry or cure to a solid coating.A pressure or thermal adhesive tape is laminated onto the solid adhesivematerial. When the relative adhesion between the electron field emittermaterial, the adhesive coating, and the adhesive tape are properlybalanced, peeling of the adhesive tape leads to the removal of theadhesive coating from the cathode assembly and the improved emission ofthe electron field emitters.

Example 25

The use of photoimagable silver, dielectric, and nanotube/silver emitterpastes in the construction of a thick film based field emissive triodearray in an inverted gate triode array in a rib-geometry provides anumber of advantages. The design of the described triode array overcomessignificant difficulties encountered with electrostatic charging inother inverted or under-gate designs. The fabrication procedure alsoovercomes difficulties related to feature alignments of the variouslayers. The use of the process of the invention for improving emissionof such a triode can be achieved as described in this Example.

An inverted-gate triode has the electron field emitter cathodephysically between the gate electrode and the anode. The cathodeassembly consists of gate electrode lines deposited as the first layeron the surface of a substrate. A layer of dielectric ribs, orientedorthogonal to the gate lines, forms the second layer of the device. Thedielectric ribs are capped with cathode conductor current feed lines.Forming the top layer of the cathode assembly is an electron fieldemitter layer deposited on the cathode conductors. The electron emitterlayer may be fabricated either as continuous lines or discontinuoussegments or dots as required by the display design. Critical dimensionsin the device include the rib width, the dielectric thickness, and theedge to edge capping of the dielectric ribs by the electron fieldemitter layer. It is very important that no electrical contact existbetween the cathode conductor and gate layers.

The following process will result in the fabrication a cathode assemblywith an inverted gate triode array in a rib-geometry using photoimagablethick films. Various modifications in the steps will be obvious to thoseskilled in the art. The process for making a cathode assembly with aninverted gate triode array in a rib-geometry using photoimagable thickfilm pastes comprises:

-   -   (a) printing on a substrate a photoimagable silver gate layer,        photo-imaging and developing the silver gate layer and then        firing to produce silver gate lines on a substrate, wherein each        silver gate line width extends well beyond the width of the        electron emitters it is to control and the gate line width        covers much of the substrate in the vicinity of the electron        emitters,    -   (b) printing one or more uniform photoimagable dielectric layers        on top of the silver gate lines and the exposed substrate and        drying the dielectric,    -   (c) printing a photoimagable silver cathode feed layer on top of        the dielectric and drying the silver cathode feed layer,    -   (d) printing a photoimagable electron field emitter layer and        drying the electron field emitter layer,    -   (e) using a photo-mask containing a rib pattern to image the        electron field emitter, the cathode feed and the dielectric        layers in a single exposure thereby achieving perfect alignment        of the electron field emitters and the cathode feed lines on top        of the dielectric ribs, and    -   (f) developing the electron field emitter, cathode feed, and        dielectric layers to produce the rib geometry and co-firing the        electron field emitter, cathode feed, and dielectric layers        under conditions that are compatible with the electron field        emitter.

The cathode assembly containing the inverted triode array is then readyto undergo the process of the invention for improving emission.

For a triode array consisting of electron field emitter dots orsegments, in step (d) a photoimagable electron field emitter layer isprinted as lines in registration with and parallel to the center of thesilver gate lines below. Alternately, a uniform photoimagable electronfield emitter layer can be printed to obtain lines of emitters in afinished triode. The electron field emitter layer may also be depositedat a later stage if it is desirable to fire the dielectric and cathodelayers in a different atmosphere and at a different temperature thanthat required for the emitters; in this embodiment anotherprint/dry/image/develop/fire sequence is necessary to fabricate theelectron field emitters on top of the cathode feed capped dielectricribs. This second imaging step does require critical alignment of thephoto-mask in registration with the preformed dielectric ribs.

The above process illustrates how perfect registration of the electronemitter, cathode feed, and dielectric features can be achieved withoutany critical alignment step using photoimagable thick films. Mostimportantly, this process prevents short formation between the silvergate and the electron field emitter layers while at the same timeminimizing exposed dielectric surfaces around the emitters. Thelikelihood of electrostatic charging during operation is thereforegreatly reduced. In addition, since the electron field emitter layer islocated on top the device, the use of an adhesive material to practicethe process of the invention for improving emission of the electronfield emitter material is straightforward with the inverted gate triodearray.

1. A process for improving the field emission of an electron fieldemitter comprised of an acicular emitting substance, comprising: (a)attaching particles of an acicular emitting substance to a substrate toform said electron field emitter; (b) contacting a material with saidelectron field emitter, wherein said material forms an adhesive contactwith said electron field emitter and said adhesive contact providessufficient adhesive force when said material is separated from saidelectron field emitter so that a portion of said electron field emitteris removed or rearranged thereby forming a new surface of said electronfield emitter; and (c) separating said material from said electron fieldemitter.
 2. The process of claim 1 wherein, when said material isseparated from said electron field emitter, a portion of said electronfield emitter is removed.
 3. The process of claim 2 wherein saidacicular emitting substance is acicular carbon.
 4. The process of claim3 wherein said acicular carbon is comprised of carbon nanotubes.
 5. Theprocess of claim 4 wherein said carbon nanotubes are single wall carbonnanotubes.
 6. The process of claim 5 wherein said single wall carbonnanotubes are laser ablation grown single wall carbon nanotubes.
 7. Theprocess of claim 5, wherein said carbon nanotubes are less than about 9wt % of the total weight of said electron field emitter.
 8. The processof claim 5, wherein said carbon nanotubes are less than about 5 wt % ofthe total weight of said electron field emitter.
 9. The process of claim5, wherein said carbon nanotubes are less than about 1 wt % of the totalweight of said electron field emitter.
 10. The process of claim 5,wherein said carbon nanotubes are about 0.1 wt % to about 2 wt % of thetotal weight of said electron field emitter.
 11. The process of claim 4wherein said carbon nanotubes are multiwall carbon nanotubes.
 12. Theprocess of claim 4 wherein said carbon nanotubes comprise single wallcarbon nanotubes and multiwall carbon nanotubes.
 13. The process ofclaim 3 wherein said acicular carbon is comprised of carbon fibers grownfrom the catalytical decomposition of carbon-containing gases over smallmetal particles, each of which said fibers has graphene plateletsarranged at an angle with respect to the fiber axis so that theperiphery of said carbon fiber consists essentially of the edges of saidgraphene platelets.
 14. The process of claim 1 wherein the electronfield emitter is rearranged, and little or none of the electron fieldemitter is removed.
 15. A process for improving the field emission of anelectron field emitter comprised of an acicular emitting substance,comprising: (a) attaching particles of an acicular emitting substance toa substrate to form said electron field emitter; (b) contacting amaterial with said electron field emitter, wherein said material formsan adhesive contact with said electron field emitter, wherein there isno translational motion by said material with respect to the electronfield emitter, and wherein said adhesive contact provides sufficientadhesive force when said material is separated from said electron fieldemitter so that a portion of said electron field emitter is removed orrearranged thereby forming a new surface of said electron field emitter;and (c) separating said material from said electron field emitter.
 16. Aprocess for improving the field emission of an electron field emittercomprised of an acicular emitting substance, comprising: (a) attachingparticles of an acicular emitting substance to a substrate to form saidelectron field emitter; (b) contacting a liquid material with saidelectron field emitter, wherein said material forms an adhesive contactwith said electron field emitter, and said adhesive contact providessufficient adhesive force when said material is separated from saidelectron field emitter so that a portion of said electron field emitteris removed or rearranged thereby forming a new surface of said electronfield emitter; and (c) separating said material from said electron fieldemitter.
 17. A process for improving the field emission of an electronfield emitter comprised of an acicular emitting substance, comprising:(a) attaching particles of an acicular emitting substance to a substrateto form said electron field emitter; (b) contacting a material with saidelectron field emitter before said electron field emitter is fired,wherein said material forms an adhesive contact with said electron fieldemitter, and said adhesive contact provides sufficient adhesive forcewhen said material is separated from said electron field emitter so thata portion of said electron field emitter is removed or rearrangedthereby forming a new surface of said electron field emitter; and (c)separating said material from said electron field emitter.
 18. A processfor improving the field emission of an electron field emitter comprisedof an acicular emitting substance, consisting essentially of: (a)attaching particles of an acicular emitting substance to a substrate toform said electron field emitter; (b) contacting a material with saidelectron field emitter, wherein said material forms an adhesive contactwith said electron field emitter, and said adhesive contact providessufficient adhesive force when said material is separated from saidelectron field emitter so that a portion of said electron field emitteris removed or rearranged thereby forming a new surface of said electronfield emitter; and (c) separating said material from said electron fieldemitter.
 19. In the fabrication of a field emission triode, a completelyscreen printed field emission triode or a lighting device that comprisesan electron field emitter comprised of an acicular emitting substance, aprocess for improving the field emission of the electron field emittercomprising: (a) attaching particles of an acicular emitting substance toa substrate to form said electron field emitter; (b) contacting amaterial with said electron field emitter, wherein said material formsan adhesive contact with said electron field emitter and said adhesivecontact provides sufficient adhesive force when said material isseparated from said electron field emitter so that a portion of saidelectron field emitter is removed or rearranged thereby forming a newsurface of said electron field emitter; and (c) separating said materialfrom said electron field emitter.
 20. The process of claim 1, 15, 17, 18or 19 wherein the material is applied in liquid form.
 21. The process ofclaim 1, 15, 16, 18 or 19 wherein the material is thermally softenedpolymer film.
 22. The process of claim 21 wherein the material isselected from the group consisting of an acrylic film, anethylene/acrylic elastomer film, a block copolymer film and an ionomerfilm.
 23. The process of claim 15, 16, 18, 19 or 20 wherein, when saidmaterial is separated from said electron field emitter, a portion ofsaid electron field emitter is removed.
 24. The process of any one ofclaims 15˜19 or 20 wherein said acicular emitting substance is carbonnanotubes.
 25. The process of claim 24 wherein said carbon nanotubes aresingle wall carbon nanotubes.
 26. The process of claim 25 wherein saidsingle wall carbon nanotubes are laser ablation grown single wall carbonnanotubes.
 27. The process of claim 24 wherein said carbon nanotubes aremultiwall carbon nanotubes.
 28. The process of claim 24 wherein thecarbon nanotubes are single wall carbon nanotubes and multiwall carbonnanotubes.
 29. The process of claim 24 wherein said carbon nanotubes areless than about 9 wt % of the total weight of said electron fieldemitter.
 30. The process of claim 15, 16, 18, 19 or 20 wherein theelectron field emitter is rearranged, and little or none of the electronfield emitter is removed.
 31. The process of claim 15, 16, 18, 19 or 20wherein the acicular emitting substance comprises carbon fibers grownfrom the catalytical decomposition of carbon-containing gases over smallmetal particles, each of which said fibers has graphene plateletsarranged at an angle with respect to the fiber axis so that theperiphery of said carbon fiber consists essentially of the edges of saidgraphene platelets.